Equinox (celestial coordinates): Difference between revisions

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In [[astronomy]], '''equinox''' is either of two places on the celestial sphere at which the [[ecliptic]] intersects the [[celestial equator]]. In contrast to the common usage of spring and fall equinox, it is a place in space and not a moment in time.<ref>{{cite book |last= |first= |authorlink= |title=Astronomical Almanac for the Year 2019 |url= |accessdate= |year=2018 |publisher=United States Naval Observatory |location=Washington, DC |isbn=978-0-7077-41925 |page=M6 }}</ref><ref>{{cite book |last=Barbieri |first=Cesare |authorlink= |title=Fundamentals of Astronomy |url= |accessdate= |year=2007 |publisher=Taylor and Francis Group |location=New York |isbn=978-0-7503-0886-1 |page=31 }}</ref>
In [[astronomy]], '''equinox''' is a moment when the [[vernal equinox|vernal]] point, [[celestial equator]], and other such elements are taken to be used in the definition of a [[celestial coordinate system]]. The position at other [[equinox]]es can be computed by taking into account [[precession]], [[nutation]], and [[aberration of light|aberration]], which directly affect e.g. [[right ascension]] and [[declination]].
Contrastingly, ''[[epoch (astronomy)|epoch]]'' is a moment for when a given position of an [[astronomical object]] is valid. The position at other epochs can be computed by taking into account [[proper motion]], [[parallax]], and [[orbital elements]].
In the case of orbital elements for objects within the [[Solar System]], only a few of the classical orbital elements are affected by a switch of equinox: the [[longitude of the ascending node]] and (to a much lesser extent) the [[orbital inclination|inclination]]. If another set of orbital elements are used, such as the position and velocity vectors for a particular epoch, all components can be affected by a switch of equinox.